Wheels, Wheels, Wheels! We all need them and we all have them. However,
over 70% of racers don't understand how important the choice is to our racing
performance. In the micro sprint world there are dozens of choices when
it comes to brands of wheels. With the limited amount of power available,
it is the most critical choice often overlooked. Out of all
the wheel choices, I consider there to be a few good choices and a
few bad choices. Every wheel has some disadvantages and some advantages. There
are those that wash off well, those that shine, those that bounce off
walls and those that endure wrecks like rubber balls and last forever. Some
wheels create horsepower, handling and speed that others are incapable
of achieving. Creating horsepower, handling and speed are
the most important issues when your ultimate goal is a performance
advantage. Spending $1000's of dollars on motors, adjustable shocks,
and weight jackers is pointless when you bolt pounds of extra weight
to the most critical points on your suspension. There are many small drivers
who need to add weight to make legal limits and are not concerned with
wheel mass. They are foolish. Every pound of rotating weight is equal
to 7-9 pounds of in car weight. For example, if you have a wheel that
weighs 2 lbs. more then your competitors x 4 wheels, your car believes
it has an extra 70+ pounds in the seat. It also critically affects your
unsprung weight. For every bump that a heavier wheel incurs, it looses milliseconds
of handling that equal tenths of seconds at the flag stand. Please remember
when considering purchasing wheels, what makes you fast and what your retailers sell
may be completely different. There are companies in the industry that have
flourished in recent years selling high margin products that do not necessarily
make you faster. Please do not believe all wheel manufacturers when they
claim to be 3x lighter. Weigh them! Chances are they are lying,
because they also know how important rotating weight is. If you don't
weigh it you will never truly now. I can promise you this; the
fastest racers in the industry learn to think for themselves.
Excess wheel mass creates rotational inertia as well as excess un-sprung weight which is a critical factor to your chassis performance and handling. Not only do you speed this mass up, but you also have to slow it down. It is the most important place on the car to consider weight. The less rotational wheel mass you carry the better your shocks will perform, your steering will also be more responsive, your motor will produce more hp to the ground and your brakes will even perform better as you also need to slow that rotational mass down when braking. The torsion arms will benefit from less rotating mass and unsprung weight. Once again, weight is the most important consideration when buying a wheel. If you haven't weighed it, you truly don't know.
Offsets & Widths
For the most part, wheel offsets are not a big thought around the pits. Most racers use what their chassis manufacturer has suggested or sold with the car, which is a safe way to start. The most commonly sold RR wheel by Keizer is a 10x13x5 BL wheel. For rookies, that equates to a 10"diameter x 13" wide x 5" inside half and an outer beadlock. We see narrower wheels in the eastern part of the country because of width rules. The central states do run as wide as 14” & 15" wheels. A lot of it comes down to preference. The width plays factors with overall role out stagger numbers and sidewall deflection. It also changes how the tire reacts to certain surfaces. On a heavy track I prefer a wider wheel, as it holds the tire in place and keeps the car free. In other words, it does not allow the load point to change. On a slick track, a narrow wheel can perform better as it allows the car some extra side bite. As the tire roles under the chassis it can change the load point, gain side bite and allow some extra spring rate. Together these characteristics all have a tendency to tighten your racecar.
Offset is the second factor which changes how your car can react. It is a pretty simple and straight forward concept. Again, in changing offsets you are changing the load point on the wheel. This can be easily seen through your tire wear. The fastest 600cc racers I know pay close attention to tire wear as it is an indicator to your load points. Assuming wheel width and position stay the same, the deeper offset or inner half, the tighter the chassis will act. This is done by moving the load point to the outside of the RR wheel. Likewise, the smaller the offset or inner wheel half, the looser the chassis will be. This is done by moving the load point to the inside of the RR wheel and not allowing for side bite. Here again, a deep inner half will create tire wear on the outside of the tire. A shallow inner half will create tire wear on the inside of the tire. Much like moving your LR tire in or out can produce different wear patterns and load points, the same can be true with different offset wheels. Keep it simple. Moving the wheel out loosens the car up, while moving the wheel in will tighten the right side offset or wheel position.
The left rear for the most part is universal in the micro sprint world. 80% of the cars run a 10x8x3 BL, that's a 10" diameter x 8" width x 3"inside offset or half. With the width of the chassis of today, there is not a lot of room on the axels for much more of an offset or even movement. Tire options do give us a 10" wide left rear choice. This usually requires the offset to remain the same if a 10x10x3 BL wheel is used. If you do have room to move the LR, simply moving the tire out will tighten the car up and give you drive off the corner as moving it in will free you up and help the car turn during entry. The bottom line is rear offsets do affect handling and should be kept in consideration when changing wheel combinations.
Front wheels are where we see the biggest variance in today's wheel options. As front axles are becoming wider to help with handling performance, the chassis manufacturers are pulling the wheels over the kingpins to help them steer and acquire a better ‘scrub’ radius. Some manufactures will use a 10x7x3. Many manufactures suggest a 10x6 wide wheel with either a 3" or 4" inside half. If you are uncertain of your wheel makeup, don't hesitate to ask someone. Keizer Wheel representatives know most chassis setups and will help you decide what is the best and most cost effective way to achieve what you are trying to do. Have a handling problem? Sometimes a simple adjustment in wheel offset or width can change the handling of the car dramatically. Please keep in mind, your front load points are affected by offset the same as your rear.
The following is an essay from a top U.S. University who competes in a formula type racing program. F-SAE is a engineering based program that focuses on the design and build of 600cc formula cars. Remember, these are cars that run on smooth uninterrupted surfaces. Yet they regard rotational mass and unsprung weight as top concerns when building a racer to achieve peak handling. The essay is very interesting and can help you better understand the importance of rotational mass and un-sprung weight. Look to the bottom of the essay as it is explained best in laymen's terms.
A Complete Description of the Effects of Rotational Wheel Mass in Both Engineering and Laymen's Terms
By: Ryan Siefring
University of Dayton
Formula SAE Team Leader
Date Due: March 11 2006
To the Engineer
The amount of wheel mass is a critical factor when selecting a wheel to use. The selection of a set of wheels with the minimal amount of mass possible is of the utmost importance. Wheel selection is a central part of building any performance machine because it affects such characteristics as acceleration, deceleration “braking”, and handling of the vehicle. In the following sections of this report you will see exactly how mass drives all these factors mentioned above.
Newton ’s second law will be the basis for the explanation of how this mass affects acceleration. This law states that force equals mass times acceleration, force is also proportional to mass, and there is also a rotational equivalent for mass: the moment of inertia, I, which represents an object’s resistance to being rotated. Using the three rotational variables, we can arrive at a rotational equivalent for Newton ’s Second Law: Torque (T) equals moment of inertia (I) multiplied by rotational acceleration ( á) . This equation can be rewritten to state that:
á = T/I
From the equation above, note that the acceleration is inversely proportional to the moment of inertia. This means that the lower the moment of inertia the higher the value of acceleration will be. Also, because deceleration “breaking” is acceleration in the opposite direction the equation shows how stopping power is affected by this same equation. From this equation we can see that the lower the moment of inertia the better.
Now that we understand how critical the moment of inertia is of a wheel, let’s look at how the moment of inertia relates to mass. Mass is a property of a physical object that quantifies the amount of matter it contains . However, what we are interested in is the dynamic affects of mass which relates to the moment of inertia of that object. For objects made up of many particles, the moment of inertia is the sum of all the moments of inertia for the individual particles. Real objects are indeed made up of many particles, so many which treating them individually is a daunting task. Engineers have figured out methods to determine this inertia value. There are three common ways in which they measure this moment of inertia. Mathematically summing up this inertia value can be accomplished through integral calculus or by using the equations below. Solid modeling may also be used if a three dimensional electric copy is available. Or if the object is not too large, it is possible to measure the moment of inertia by applying a known torque and measuring the angular acceleration. For certain very simple objects we may determine the moment of inertia by reasoning alone, based on the m*r 2 moment of a single particle I = M * R 2.. This is the real challenge involved in the rotational version of Newton ’s Second Law, which is sorting out the correct value for the moment of inertia.
Mass of a wheel also affects the handling of a vehicle in the affect of unsprung weight. Unsprung weight is any weight that is not supported by the springs on a vehicle. In such a dynamic event this mass is free to move up and down the changes in the roads surface. Because the wheels are part of this unsprung weight we will look at how this mass affects the vehicle. This mass is related to kinetic energy needed to keep the wheel gripping the surface. The more energy that is needed the harder it will be for the suspension to maintain traction with the surface in question. Below is the equation for this relationship.
Ki=1/2*W 2*Mi*Ri 2
K= Kinetic Energy
W = angular speed
Mi= mass
Ri = radius from the axis
From this relationship we see that the less the mass of the wheel, less energy is required to control it. In this case the wheel is part of the unsprung weight and unsprung weight is the weight under the springs which moves up and down as the vehicles hides over uneven roads and leans in the corners. For example, this is important because, the greater the unsprung mass the harder the suspension components have to work when a wheel hits an irregularity in the road. The more mass in a tire the greater the kinetic energy is created when it hits an irregularity in the road. Both raised sections and bumps reduce the tires ability to have full possible contact pressure when these vertical accelerations are experienced. This is how handling is affected by the mass of a wheel, the lighter the wheel the more effective contact the tires will have with the road because it takes less energy to keep it in contact with the roads surface at all times.
In conclusion inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily. Also, because mass is a function of Inertia it also affects the vehicles handling. From this information one would want to select the lowest weight wheels possible.
To the Weekend Warrior
A good rule of thumb is to use a wheel that is as light as possible without sacrificing strength and only increase the diameter if extra disc brake clearance is needed. The universal understanding that heavier is slower works well for this situation. There is always performance to gain when weight is lost and the part does not break. All too often people forget this when selecting a wheel; sadly, there are people who pick their performance wheels on looks and lies alone. Sometimes these new wheels end up weighing many more pounds than their factory wheels. It is an understandable mistake when one is not properly educated on this subject. For the logic that, “what is a few more pounds my car already weights 750 pounds” is an easy misconception to make if there is no background in this subject. However, a few more pounds of weight per wheel is no trivial matter for each wheel is part of the drive line of the vehicle and is also part of the vehicles unsprung weight. Let’s now relate how this extra weight really affects the performance of the vehicle. Imagine a bicycle wheel and the “wheel of fortune” on The Price is Right. Which one do you think is easier to spin? The bicycle wheel is much lighter than the other and therefore takes less work to spin it. Think of how much faster you can get it to rotate. Although, this might not be as exciting as “The Price is Right wheel” you should see how much harder your vehicle also has to work when you are trying to put the power to the road with a heavier wheel. From this observation it is understandable that a heavier wheel is much harder to rotate than the lighter wheel and this weight affects many important aspects of performance in ones vehicle.
Now let’s look at how this weight also affects the handling of your vehicle. It is also important to not only accelerate your wheels, but to also keep them firmly planted on the road. For this reason it is also important for them to be as light as possible.
For example imagine this scenario,
- Place your arms our stretched perpendicular to your body and have someone place a bowling ball in your hands. In this case your arms represent the suspension arms and our muscles the springs. Imagine the bowling ball as the rim and tire combination.
- Now toss the bowling ball up into the air and catch it. Feel the amount of effort that it take to counter act it s weight.
- Next attempt this experiment with a tennis ball, throw it up into the air and feel how easy it is to stop.
Take note of the differences between the two situations. It was the one with less weight that was easier for you to control. The same goes for your vehicle, a lighter wheel in all situations will be easier for the suspension to control in any situation. The lighter wheel allows your suspension to better react to any bump of dip in the roads surface. This allows for more contact with the road.
Now you know how to make a good selection when purchasing a new set of rims. The next time you purchase a set of rims, Look for ones with a low amount of weight and do not make a selection on looks alone. For weight will greatly affect your performance of your vehicle in all conditions, acceleration, breaking, handling, and even gas mileage.
Works Cited
Wong, J. Y. Theory of Ground Vehicles 2ed. United
